Antenna diversity receiving systems having an antenna device including a plurality of FM antennas are known. These systems have a controlled switching circuit for sequentially switching through one of the plurality of FM antennas via an antenna cable to a receiver upon reception of a switching control signal. The receiver has a multipath detector coupled to a pulse generator for generating a pulse signal at the detection of multipath interference. Such a receiver is described in European Patent Application number 0 792 031 and is specifically suited to be used in vehicles.
The reception of a wanted RF broadcast transmitter signal may be disturbed or otherwise deteriorated by various phenomena, such as multipath reception and/or adjacent channel interferences. In general, multipath reception is caused by signal reflections at and/or against environmental physical obstacles, such as mountains, trees, buildings, fences and the like. Due to such signal reflections an RF broadcast signal may arrive at a certain reception location through different signal paths, i.e. in different amplitude and phase conditions. The summation of these multipath signals at the receiver antenna results in unpredictable signal amplitude and/or phase distortions, most often effectuating in part or complete cancellation of the useful RF reception signal. These signal cancellations, being referred to as signal dips, strongly depend on the RF carrier frequency of the received RF broadcasting signal and on the location of reception.
Signal dips severely deteriorate the desired RF broadcasting signal and also the overall signal reception quality. However, a relatively small shift in the position of the antenna receiving the desired RF broadcasting signal may already suffice to strongly improve signal reception quality. This solution is used in antenna diversity receiving systems of the above type to avoid reception of multipath distorted RF signals. In such antenna diversity receivers use is made of two or more antennas mutually spaced apart and coupled to an RF input of a receiver. Only the antenna having the best local receiving conditions with respect to the other antenna(s) is actually connected to the RF receiver input. This antenna, also referred to as the actual antenna, is effective in the reception and supply of the desired RF broadcasting signal through the antenna cable to the receiver as long as the multipath distortion at the actual antenna remains smaller than a certain predetermined multipath threshold level. As soon as the received multipath distortion exceeds the multipath threshold level, the RF signal supply to the receiver is changed from the actual antenna to another antenna positioned at a location with better receiving conditions. With proper control of the controller circuit, the receiver is continuously optimized for minimum multipath reception.
Present antenna diversity receiving systems have a multipath detector with an output coupled to a pulse generator for generating a pulse signal at the detection of multipath interference. This pulse initiates a proper switching operation resulting in a switch over of an RF broadcast signal from one antenna to a subsequent antenna. The pulse signal is supplied through the antenna cable to the controller to initiate the antenna switching operation. This switching operation is repeated if the RF broadcast signal received at the subsequent antenna also appears to be affected by multipath distortion exceeding the multipath threshold level, until an RF broadcast signal is actually received which is not affected by such multipath distortion.
The antenna cable carries RF broadcast signals (from the antenna device to the receiver) as well as pulse signals (in the opposite direction). These signals mutually interfere and in particular the pulse signals effect the useful FM RF broadcast signals and may become noticeable in the reproduced audio signals.
There is thus a need to simplify existing antenna diversity receiving systems allowing for a cost effective implementation thereof, while providing optimal signal reception. There is also a need for an antenna diversity receiving system using a single antenna cable for the transmission of both useful FM RF broadcast signals and pulsating switching control signals to secure an accurate detection of these pulsating switching control signals and to prevent the pulse signals from disturbing the processing of the useful FM RF broadcast signals in the receiver. There is a further need for a system to allow for the reception of various types of RF broadcast signals, in particular both AM and FM RF broadcast signals while preventing mutual interference between the various signals passing one and the same antenna cable, from occurring.